Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Problem with gravity and masses

  1. May 5, 2008 #1
    hi guys,

    I was playing around with Newton Equation F=Gm1m2/r^2

    I separate Fr^2 = Gm1m2 . This is bugging me. Fr^2 is a constant for a given 2 bodies(i call Z) of masses. That means that no matter what the distance r in between the two bodies, Z doesn't change. So i take my thought alittle further. Suppose the two bodies are almost infinite small with say ... 5 and 10 mass. so Z value is 50G. Since no matter how close u put them together, the value Z doesnt' change. I put them so close together that they almost like 1 body with a total mass of 15mass. Now I imagine what if they are 7 and 8 mass. The Z value is 56G. I put them throught the same process and let them come so close together that they're almost 1 body with 15mass. Now it looks like I have two identical bodies with different Z value. It seems disturbing.
  2. jcsd
  3. May 5, 2008 #2


    User Avatar
    Homework Helper

    but "Z" doesn't mean anything unless the bodies are seperated and if you seperate them in two different ways it shouldn't be surprising that you get two different "Z values".
  4. May 5, 2008 #3
    I guess the question is... what does Z means. lol I imagine r^2 as a square and F go up and down to have a constant volume. Though I don't know what it means, but it seems unique.
  5. May 6, 2008 #4


    User Avatar
    Homework Helper

    it doesn't mean a thing besides G*M_1*M_2... which has only one value for a given M_1 and M_2 but may be different for different values of M_1 and M_2... even when M_1 + M_2 =M_tot is fixed, as you see in your example. but that, too, is meaningless.
  6. May 7, 2008 #5

    Fine! I'll keep it as an souvenir. lol
  7. May 7, 2008 #6
    Z is a function of r. You can't say that Z is independent of r...basically, when r changes, Z changes. It's just like saying that y=Gx^2 but y doesn't change while x can, it's just completely nonsensical.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Problem with gravity and masses